This disclosure is generally related to high speed wireless packet-based networks and devices. In particular, this disclosure is related to interference between devices operating in a time division duplex (TDD) network and a frequency division duplex (FDD) network. Examples of TDD and FDD networks include networks operating on wireless technologies such as Worldwide Interoperability for Microwave Access (“WiMAX”) technologies and Long Term Evolution (LTE) technologies, or other “fourth generation” (4G) wireless technologies.
In many geographical regions, considering the nature of the regulatory environment and economic benefits, it may sometimes be advantageous to deploy dissimilar wireless networks to fully utilize a wireless operator's frequency spectrum including one or more paired frequency bands (i.e., frequency bands with separate carrier frequencies for uplink and downlink communications), and one or more unpaired bands (i.e., frequency bands in which both uplink and downlink communications are provided by the same carrier frequency). By deploying a FDD system using paired frequency spectrum allocations and a TDD system using stand-alone unpaired frequency allocations, wastage of valuable spectrum may be avoided.
However, when a TDD system operates on carrier frequencies adjacent to that of an FDD system, the possibility of severe interference may arise. In particular, severe interference may exist for the interference between base stations of the two networks, i.e., between TDD base station (BS) and FDD BS, and between TDD mobile station (e.g., WiMAX MS or LTE TDD user equipment (UE)) and FDD UE. For simplicity, the term “BS” is used throughout this disclosure to represent, for example, a TDD base station (e.g., a WiMAX base station or an “enhanced Node B” (eNB) as known in LTE TDD systems), as well as an FDD base station (e.g., LTE eNB). Also, throughout this disclosure, the term “MS” is used to represent a TDD wireless device, and the term “UE” is used to represent an FDD wireless device. Various methods, e.g. site-planning or special filters installed on base stations, may be employed to effectively mitigate the interference between base stations, e.g., the interference case of TDD BS-FDD BS, and interference between a base station and its associated mobile device(s), e.g., the interference cases of FDD UE-TDD BS and TDD MS-FDD BS. Yet, the interference between mobile devices each associated with a different base station, e.g., interference between TDD MS and FDD UE may be considered to be the most challenging case that poses a relatively greater risk for ensuring coexistence between dissimilar TDD and FDD networks.
Severe performance degradation may occur when a TDD MS and an FDD UE operating concurrently are located in a closed and small space, e.g., a café, a conference room, etc. Additionally, interference between the TDD MS and FDD UE may be severe when the MS and UE are located at an edge of a cellular region (or at a cell edge) which is served by co-located TDD and FDD base stations (i.e., base stations that are located with close geographical proximity and have respective coverage areas with significant overlap), or at the cell edge of two adjacent cells served by non co-located TDD and FDD base stations. In either of these cases, the interference problem between TDD MS and FDD UE is exacerbated, as the received downlink signal of FDD UE (or TDD MS) is typically quite weak, while the interfering TDD MS (or FDD UE) transmits an uplink signal typically at a level close to a maximum output power of the TDD MS (or FDD UE). From the perspective of user-perceived service quality, cell-edge mobile devices in high-density indoor environments, e.g. café, airport etc., are particularly vulnerable to severe performance degradation caused by TDD MS-FDD UE interference. This is because stationary mobiles in such environments are likely to transmit/receive persistently for long periods of time, which may result in prolonged service disruption due to strong interference. This persistent interference problem for indoor hotspot mobile devices could be better appreciated by comparing to that of outdoor mobile devices, which are likely to be mobile and therefore, may not stay in close proximity with each other for long periods of time. Thus, unlike the interference experienced by indoor hotspot wireless TDD or FDD devices, the interference and therefore, the performance degradation of outdoor TDD or FDD devices may be transient.
What is needed is a system and method to detect and mitigate interference between a TDD and an FDD network, particularly interference between mobile devices, e.g., interference caused at an FDD UE by a TDD MS, or interference caused at a TDD MS by an FDD UE operating in the TDD and FDD networks, respectively with adjacent and/or overlapping frequency bands.